Calcium bentonite clays, i.e., clay in which the principal exchangeable cation is a calcium ion, are also referred to as sub-bentonites, or calcium montomorillonites. Generally, these are hydrated aluminosilicate crystalline minerals. Usually, magnesium proxies for some of the aluminum in the crystals of the clay. Iron content varies with clays from different deposits.
For many years, selected bentonite source clays have been treated on a commercial scale with acids to leach aluminum from the structure. The acid leaching has been practiced to produce bleaching earths, cracking catalysts and reactive pigments for the CF paper for carbonless copying paper systems. See U.S. Pat. No. 4,405,371 Sugahara et al, U.S. Pat. No. 3,622,364, Sugahara et al and U.S. Pat. No. 4,118,247, Marchetti et at. It has been reported (CA130:5055) that mixtures of acid leached bentonite and kaolin are useful in formulating coating pigments for inkjet printing. Reactive pigment systems for producing carbonless CF sheets that use an acidic resin plus porous pigments are disclosed in U.S. Pat. No. 5,350,729 Londo et at.
The starting clays which are used to produce heretofore known forms of acid leached bentonites typically contain approximately 20% alumina (based on the dry weight). The aluminum in bentonites is in octahedral and tetrahedral bonding structures. Acid dosages of about 40-50 gm of 96% H.sub.2 SO.sub.4 /100 gm clay are typically used. Alkaline earth and alkali metals are removed. The clays are usually leached to a residual aluminum content in the range of about 10-15 wt. %. The extent of leaching varies inter alia with the intended use of the leached clay. However, in general practice, both octahedral and tetrahedral aluminum remain in the solid residue which, when studied by XRD, exhibits lines characteristic of the clay crystals. The acid treated clay is invariably washed to remove soluble salts and entrained acid. While sulfuric acid is usually the acid of choice, other acids such as phosphoric and oxalic acids have been proposed.
It is known that repeated sulfuric acid leaches, resulting in extractions in excess of those used in the typical commercial prior art practice, can produce siliceous residues with essentially no aluminum. The porosity (surface area and pore volume) can be severely destroyed by such practice. This may explain why exhaustive leaching to remove virtually all aluminum has not been practiced commercially.
Acid-activated bentonites had been used as reactive pigments for several decades for paper products, in particular for use as a porous pigment in developer sheets for carbonless copy paper manufacture. The acid-leached bentonite was used with an oily solution of normally colorless leuco dyes encapsulated in microcapsules to develop colored images. In the case of U.S. Pat. No. 4,405,371, Sugahara et al proposed to use a relatively highly leached bentonite. The bentonite was leached by H.sub.2 SO.sub.4 or HCI to such a degree that the SiO2 content was about 82-96.5 wt. %, preferably 85-95 wt. %. The acid-leached bentonite was characterized by its loss of X-ray crystallinity, regardless of its aluminum content or structure. However, it was noted that the acid-leached bentonite had a relatively low BET surface area, about 180 m.sup.2 /g. While acid treated bentonites are used as reactive pigments in carbonless paper in Europe, in the U.S. markets acidic phenolic resins are now used as the color developer in carbonless paper.
It is known that surface characteristics of paper (or any other printing surface) play a large role in how ink will be received and appear after application to the printing surface when using inkjet printing. Thus, varying print appearances can be expected depending on whether the surface ink is being applied to is uncoated or coated paper. Printing on uncoated paper results in low qualify printing while printing on coated paper results in a higher quality print albeit of varying quality according to the nature of the paper coating composition.
Two of the more important characteristics to be controlled in color ink jet printing are depth of penetration and feathering or bleeding of the ink when applied to the paper. Too deep of a penetration results in poor color intensity. Bleeding results in poor printing definition. A more recent criterion, is to control the contact angle of the various inkjet colors (i.e., cyan, magenta, yellow and black) in a manner that the inks will substantially have the same contact angle when applied to the coated paper. When the contact angles of the various inks are substantially the same, the appearance of the ink colors are more uniform, i.e., one color does not appear more dull or more bright than another color.
A common component of inkjet paper coating compositions of the prior art is porous silica. While silica is an effective paper coating constituent, it is more expensive than clay-based pigments and has severe rheological limitations such as in the amount of coating solids and Brookfield viscosity. Thus, there is a need in the art for less expensive pigments which do not sacrifice printing qualities and desirably improve rheological properties. Furthermore, the silica-based paper does not satisfy the criteria for carbonless copy paper.
U.S. Pat. No. 446,174 is directed to a method of inkjet recording and identifies desirable properties of substrates or coatings for satisfactory color inkjet printing.
This patent purports that maintaining an R.sub.f value(ratio of traveling distance of dye to that of the solvent in the aqueous ink) of less than 0.59 produces images of high quality having high ink absorption and image density.
U.S. Pat. No. 4,792,487 describes inkjet recording medium coatings containing a high swelling montmorillonite clay and silica. Formulations include various polymeric binders including polyvinyl alcohol.
While there are many grades of coated paper intended for use as developer for carbonless copy paper and a variety of grades of coated paper useful for inkjet printing, these coated sheets cannot be used interchangeably. Specifically, most commercially available grades of paper for inkjet printing fail to produce a sufficiently intense image when the paper is used as the CF (developer) component of carbonless copying paper. On the other hand, commercially available CF sheets marketed for in carbonless paper cannot be used for inkjet printing because color clarity and definition are not adequate.